I'm posting this only because it seems so baffling and because someone else might run in to the same thing.
This morning my wife needed to access an iCloud file from her MBA still on Catalina 10.15.7. The original file was on her M1 iMac sitting right next to the MBA and connected to the same WiFi network.
But, iCloud refused to sync on the M1 iMac, so she couldn't access the file from her MBA.
She tried AirDrop, enabled on both machines for "Everyone". But, Airdrop simply didn't work at all on the M1 iMac. It could neither receive nor send files. (Nothing showed up in the Airdrop window on the Finder.)
I suggested plain old file sharing. She has File Sharing enabled via both AFP and SMB on both machines with read/write enabled for her only. But, when I clicked on the Network link in the M1's Finder, the only machine that showed up was a very old MBA that is currently turned off. My iMac didn't appear. Her MBA didn't appear. None of the SMB enabled Raspberry Pies appeared.
I was able to connect from the M1 to the MBA by using Connect to Server. But she'll never be able to do that on her own.
Her MBA could see all the file sharing machines on the network: The Raspberry Pies, my iMac, her M1 iMac. (But not the switched-off ancient MBA.)
And, now this is weird, when I switched to my account on the M1 iMac, all the networked machines showed up, instantly.
So, I switched back to her account, logged out and then back in again.
And, as if by magic, everything worked again. Network discovery found all the machine on the LAN. iCloud synced up right away. Air Drop worked again.
My wife calmed down and stopped growling about how "nothing works" (my usual refrain) and a pleasant calm settled on our household.
This morning my wife needed to access an iCloud file from her MBA still on Catalina 10.15.7. The original file was on her M1 iMac sitting right next to the MBA and connected to the same WiFi network.
But, iCloud refused to sync on the M1 iMac, so she couldn't access the file from her MBA.
She tried AirDrop, enabled on both machines for "Everyone". But, Airdrop simply didn't work at all on the M1 iMac. It could neither receive nor send files. (Nothing showed up in the Airdrop window on the Finder.)
I suggested plain old file sharing. She has File Sharing enabled via both AFP and SMB on both machines with read/write enabled for her only. But, when I clicked on the Network link in the M1's Finder, the only machine that showed up was a very old MBA that is currently turned off. My iMac didn't appear. Her MBA didn't appear. None of the SMB enabled Raspberry Pies appeared.
I was able to connect from the M1 to the MBA by using Connect to Server. But she'll never be able to do that on her own.
Her MBA could see all the file sharing machines on the network: The Raspberry Pies, my iMac, her M1 iMac. (But not the switched-off ancient MBA.)
And, now this is weird, when I switched to my account on the M1 iMac, all the networked machines showed up, instantly.
So, I switched back to her account, logged out and then back in again.
And, as if by magic, everything worked again. Network discovery found all the machine on the LAN. iCloud synced up right away. Air Drop worked again.
My wife calmed down and stopped growling about how "nothing works" (my usual refrain) and a pleasant calm settled on our household.
